A STUDY OF SORUS AND SPORE ONTOGENY IN UREDINIA OF HEMILEIA VASTATRIX BY LIGHT AND SCANNING ELECTRON MICROSCOPY

JOHN WILLIAM MCCAIN, Purdue University

Abstract

Hemileia vastatrix Berk. & Br. (Uredinales), cause of orange coffee rust, produces both uredinial and basidial sori. The rest of the life cycle is unknown. The uredinial sori have been variously interpreted as swollen sacs, collections of hyphae, or specialized groups of cells. They emerge from the coffee leaf through stomata and bear spores outside the leaf. Urediniospore infection produces a radially-expanding thallus that bears numerous sori and can be divided into zones. At the outer edge of the thallus is a zone of narrow pioneer hyphae colonizing the host tissue, followed by a zone where the first few haustoria are formed, and a zone of thicker, nutritive hyphae and numerous haustoria. The reproductive portion of the thallus includes a zone of sorus initials ("protosori") developing in substomatal chambers and a band of sori that have emerged from stomata but still bear immature spores, bordered by a zone of mature sori. However, sori occupy only about one-third of the host stomata. As the thallus expands, these zones spread outwards, leaving in the center of the thallus an area of senescent sori and hyphae. Protosorus development begins when two to four hyphae intertwine in the substomatal chamber. A layer of isodiametric cells is produced, distinct from the constituent hyphae. However, some protosori develop in mesophyll cavities that are not beneath stomata and must grow into the correct position. Special cells arise from the isodiametric protosoral cells, extend through the stomata, and produce urediniospores and probasidia at their tips. Multiple spore buds prove that these are sporogenous cells. Two or three, later seven or more, sporogenous cells exit a stoma as a tight fascicle. The developing sori are covered by a mucilaginous matrix. Urediniospores are extruded from the tips of the sporogenous cells in a spiral fashion. These are sympodioconidia, maturing acropetally. Spines are only on the upper surface of spores and appear when spores are 1/4-1/3 full size. Each sorus produces 30-50 urediniospores. Other genera of rust fungi also produce suprastomatal sori. Desmella aneimiae has a protosorus similar to Hemileia but produces one sorus per thallus from thickened "feeder" hyphae.

Degree

Ph.D.

Subject Area

Plant pathology

COinS